Controlled pollination of forest trees, such as conifers, is used to produce families of known parentage for progeny testing and advanced-generation breeding. Breeding conifer trees (e.g. loblolly pine trees) quickly after selection has been a challenge since the inception of tree improvement programs in the 1950's (see e.g., Zobel and Talbert, Applied Forest Tree Improvement, Wiley, New York, 505 pp. (1984). Grafts made on one-year-old rootstock in the field do not produce enough strobili (flowers) for breeding for 5 or more years. In the 1970's, techniques for greenhouse breeding were developed that shortened the time to producing strobili through the use of various induction techniques such as hormones, root pruning, water stress, girdling and out-of-phase dormancy (Greenwood, M. S., “Flower Stimulation Techniques for Loblolly Pine (Pinustaeda L.),” in Proceedings, 3rd World Consultation on Forest Tree Breeding: CSIRO:1031-1042 (1977), Greenwood, M. S, Science 210:443-444 (1978)). While effective at reducing breeding time, greenhouse breeding is expensive and has been replaced by top-grafting in loblolly pine (Bramlett and Burris, “Topworking Young Scions Into Reproductively Mature Loblolly Pine,” Proceedings: 23rd Southern Forest Tree Improvement Conference, Asheville, N.C.; pp. 234-241 (1995); Bramlett, D. L., Journal of Forestry 95(3):15-19 (1997)). By grafting in the tops of mature orchard trees, strobili can be accelerated and breeding can be completed for a given group of select trees only 2 years after grafting.
While top-graft breeding is faster and less expensive than greenhouse breeding, completion of breeding within 2 years is still a challenge because it is difficult to obtain pollen from enough males in time to pollinate all of the females before they develop past receptivity. Therefore, a need exists for effective methods to modulate strobili development in conifer trees to allow for the breeding of advanced generation selections within a shorter time period, such as within one year.